This invention generally relates to a loudspeaker, and more particularly to a loudspeaker in which a flat-plate diaphragm is driven by a plurality of magnetic drivers at a plurality of spaced-apart points.
In a flat-plate diaphragm loudspeaker, a flat-plate, usually square, diaphragm is simultaneously driven at a plurality of vibration nodes of the several vibration modes of the flat-plate diaphragm by a plurality of magnetic drivers. Simultaneous drive at vibration nodes tends to raise the frequency at which vibration of the flat-plate diaphragm breaks down into separate vibration nodes. Hence, piston motion of the flat-plate diaphragm is extended to higher frequencies and the frequency response of the loudspeaker is improved.
Voice coil bobbins having voice coils of the magnetic drivers wound thereon are connected at one of their ends to appropriate points on the rear of the flat-plate diaphragm. Dampers are connected to the body of the voice coil bobbins to stabilize the flat-plate diaphragm against all except forward and rearward motion. Since the voice coil bobbins are attached to, and vibrate with, the flat-plate diaphragm, their mass must be kept low. This prevents giving the voice coil bobbins significant strength.
Due to strength limitations in the voice coil bobbins of the magnetic drivers, the dampers are attached to the voice coil bobbins only a short distance to the rear of the flat-plate diaphragm. Due to this short distance, the dampers lack sufficent leverage to fully damp rotational vibration of the flat-plate diaphragm. Thus the flat-plate diaphragm rolls when the loudspeaker is driven. The rotational vibration may be considered as resulting from the combination of driving power of the plurality of magnetic drivers, the stiffness or compliance of the suspension unit including, for example, the damper and the diaphragm edge connection, the weight and balance of the flat-plate diaphragm owing to the distribution of adhesive used for combining the edges, bobbins and dampers and the radiation impedance of reflected acoustic waves returned to the flat-plate diaphragm by floors and walls.
Further, the plurality of magnetic drivers occupy a large proportion of the area in the rear of the diaphragm, and the distance between the flat-plate diaphragm and the magnetic drivers is necessarily smaller than that of a cone type loudspeaker. Therefore, the frame which supports the magnetic drivers requires substantial openings outside the areas occupied by the magnetic drivers. Hence, an equivalent openness ratio at the rear of the speaker, defined as the open area divided by the area of the flat-plate diaphragm, is less than that of a cone-type loudspeaker.
When the equivalent openness ratio at the rear of the diaphragm is less than 60%, the frequency response of the loudspeaker is adversely affected. In fact, the larger the openness ratio, the better the frequency response of the loudspeaker. It is difficult to increase the openness ratio since the openings can only be formed in limited areas of the frame outside the mounting area of the magnetic drivers. Furthermore, when large openings are included for increasing the openness ratio, the stiffness of the frame is reduced. As a result, the frame supporting the relatively heavy magnetic drivers is permitted to resonate with the result that the frequency response and naturalness of the reproduced sound is degraded. Accordingly, flat-plate loudspeakers are normally designed for a compromise between increasing the openness ratio, and obtaining improved frame stiffness.